Just push the rearview mirror up closer to the roof.
I know that sounds ridiculous, but I had the same problem when I first bought my VX until I realized that the mirror is on a "double jointed" mount because it articulates at the windshield and at the mirror. I just adjusted it until the mirror was tucked right up next to the roof line. It actually helped with visibility out of the rear window because now the reflection was looking over the back seat and "down" out of the rear window. Don't have it set up that way any more as I am one of those that mounted the ScanGauge above the mirror.
Tom, maybe you've already done this adjustment and are still looking to get the mirror higher.
Sorry for the TJ Jon.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless